The present invention relates to data transmission systems of the bit-parallel, byte-serial type in which more than two devices are linked to a data highway by means of individual interface circuits, hereinafter referred to simply as interfaces. Each device can act as a talker or source of data or as a listener or acceptor of data and at least one device can act as the system controller. Unless unduly long fixed periods are allowed for the transmission of bytes of data it is necessary to use so-called handshake signals to supervise the exchanges of data. Such signals are carried by control lines parallel to the data highway. These lines will be called handshake lines and carry two level signals.
In systems which only link two devices, two handshake lines suffice to ensure reliable exchange of data. The source device drives one line to a true level indicating Data Available. The acceptor device responds to this by driving the second line to a true level indicating Data Seen. The source responds in turn by driving the one line false to tell the acceptor to use the data. Having done this the acceptor drives the second line false to indicte that the data has been used. The system therefore utilizes the false to true and true to false transitions of both handshake lines, so that four meaningful signals can be interchanged in the handshake cycle using only two two-level lines. This simple scheme cannot be used when there are more than two devices linked by the highway and there may be, in any given exchange of data, a pluraity of acceptor devices. This inevitably arises in that data includes address data sent from the controlling device to the other devices. It is not possible to indicate Data Seen by all acceptor devices by one transition and Data Used by all acceptor devices by the other transition of the second line. One such transition is logically forced to occur when only one device has seen or used the data (since Data Used is logically Not Data Seen).
It has already been proposed to use more than two handshake lines, for example in various drafts of Technical Committee No. 66 of the International Electrotechnical Commission, relating to Standard Interface Systems. One such proposal, as well as the present invention will be explained with reference to the accompanying drawings.